It looks like it's only masked by keyword. My guess is you are running Gentoo stable, and this ebuild is marked unstable. There shouldn't be any problem unmasking it and giving it a try. If it doesn't work for some reason, you could also try games-misc/sdljoytest. And if you are running KDE, you can go to system setting > Hardware > Input Devices > joystick; you'll find a utility for at least testing your joystick there. Other desktops like Gnome or XFCE may have similar utilities.